This invention relates to a deflecting electromagnet used in a mass separator of an ion-injector.
In general, a mass separator for an ion-injector has a sector magnet or electromagnet for deflecting beams from an ion source at a prescribed angle. A mass separator also has a separating means for selecting ions of prescribed mass number from the beam.
A conventional mass separator or momentum analyzer having a sector magnet arrangement is shown in the plan and side view of FIGS. 1 and 2, respectively. In FIGS. 1 and 2, an ion beam 2 generated and accelerated by a source 1 is injected into a space 30 between a pair of pole pieces 31, 32 of a sector magnet 3. The ion beam is spread out in the direction of the width of a slit 11, as shown by arrow A in FIG. 1, which corresponds to the radial direction of the sector magnet. The expanded beam is then injected through an end face 51 of the pole pieces. The pole pieces are sector shaped, and re-focus the expanded beam at a separating opening 4 on the exit side. The ion beam is also expanded in the direction of the length of the slit 11 as is shown by arrow B in FIG. 2, which corresponds to the axial direction of the sector magnet before its injection into the space 30. In order to focus the axially expanded ion beam on the separating opening 4, its angle of incidence on the end face 51 must be adjusted.
The angle of incidence is adjusted by rotating the end faces 51 and 52 in the directions of the arrows C and D, respectively, by an angle of input edge variation U.sub.1 and an angle of output edge variation U.sub.2 with respect to the faces or reference planes 51a and 52a. The reference planes are at right angles with the beam 2 at its points of entry into and exit from the space 30, as shown in FIG. 1. It is well known in the optical arts that the ion beam 2 will leave as it was originally a focus on the slit 11 and the separating opening 4., when the end faces 51 and 52 are rotated in the directions of the arrows C and D with respect to the references planes 51a and 52a, respectively. It is also well known that the ion beam 2 will be moved away from focusing on the slit 11 and the separating opening 4 when the end faces 51 and 52 are rotated in the directions opposite to the arrows C and D, respectively. Accordingly, the axially expanded ion beam 2 will be focused on the separating opening 4 when the ion beam 2 is injected into and outputted from the uniform magnetic field in the space 30 between the pole pieces of the sector magnet 3 which has its end faces 51 and 52 positioned at the angles U.sub.1 and U.sub.2 to the faces 51a and 52a, respectively.
In the above conventional mass separator for an ion-injector, it is important in providing a desirable ion-optical system that the shape and angle of the end faces of the sector magnet be precisely constructed. However, in actuality, the borders of the magnetic field which is generated by the pole pieces generally do not coincide with the respective end faces of the pole pieces. As a result, the expected deflection of the ion beam is affected by the fringe field, i.e. the difference between the borders of the magnetic field and the position of the respective end faces of the pole pieces. Accordingly, in constructing a conventional ion optical system, the borders of the magnetic field are determined by the end faces of the sector magnet and the fringe field of the end faces of the sector magnet.
It is thus necessary to design the conventional deflecting electromagnet, for a mass separator or momentum analyzer, such that the borders of the magnetic field are at desirable positions and angles, so the precise magnetic field distribution is measured and processing or modification of the end faces of the sector magnet is performed repeatedly. Accordingly, a lot of time and effort are needed for the construction of the conventional apparatus. Moreover, even if the borders of the magnetic field are designed at a desirable position and angle for a chosen value of a central magnetic field, the magnetic field distribution will change by the saturation effect about the end faces of the sector magnet when the value of the central magnetic field is varied, the borders of the magnetic field will thereby shift. Furthermore, the beam west point (the beam departure or divergence point) of the ion source 1 will move in the direction of the orbit of the ion beam when the operating condition of the ion source 1 is changed, whereby the focal point of the ion beam will move in the direction of the orbit of the ion beam. Accordingly, it is very difficult to determine the shape of the end face of the sector magnet.
In addition, it has recently been desirable to provide a mass separator for an ion-injector which can inject a greater ion beam current to the target, in order to achieve greater efficiency. Therefore, it is necessary that any loss of ion beam current generated by the ion source be minimized, and that an ion-optical system which can efficiently select the ions of prescribed mass number from the ion beam be provided.